The invention relates to a rotating diaphragm for a dynamic pick-up device having a pyroelectric layer. The diaphragm rotates about an axis and periodically interrupts the thermal radiation which is incident on the pyroelectric layer. Moreover, the diaphragm comprises an electrically insulating material and a poorly conducting material.
Such a diaphragm is known from German Offenlegungsschrift Nos. 2731653 and 2731654 (corresponding to U.K. Pat. No. 1,551,520 and U.S. Pat. No. 4,227,210, respectively). Such a diaphragm may be used in dynamic pick-up devices such as pyroelectric vidicons and infrared detectors.
In order to obtain a signal with such a pickup device, for example a pyroelectric vidicon, a thermally varying picture must be incident on the pyroelectric layer. This varying picture can be obtained, inter alia, by repeatedly opening and closing a diaphragm which is situated between the observed object and the pyroelectric layer. Such a diaphragm, sometimes called a "chopper," is generally constructed as a rotating disk or cone which preferably has spiral-like parts.
Such diaphragms, when rotating immediately in front of the vidicon, however, cause interference in the video signal. The interference is caused by magnetic and electric interactions. In diaphragms consisting of insulating material, for example epoxy resin, electrostatic generation is produced due to the interaction with an electrostatic field which is always present. Processes to compensate for charges separated in this manner induce peak-shaped interference signals in the video signal. In diaphragms made of metal, eddy currents occur due to the interaction with the magnetostatic field of the focusing coil of the vidicon. The eddy currents produce magnetic fields which react on the vidicon to produce interference.
Several possible solutions have already been investigated to prevent such interference, for example the provision of a network of copper before the pyroelectric layer or an extra grounded germanium window in vidicons. On the one hand these measures are expensive, and on the other hand they are not always effective to the desired extent.